THIS invention relates to a method and apparatus for transporting a product within a diffuser of the kind used in continuous lixiviation processes, with the method and apparatus having particular application in the sugar industry.
The initial stage of sugar manufacture from sugar cane or sugar beet involves the extraction of sucrose-containing juice from the solid fibrous plant residue. In commercial plants, the first step in this process is the comminution of the plant material into small fragments, by processes such as knifing, slicing and/or shredding. The sucrose-containing juice is then extracted from the finely divided plant material by one or both of two processes:
Lixiviation, wherein the juice is washed and diffused out of the solid vegetable residue using additional liquid (water); or
Crushing or pressing, wherein the juice is expressed out of the solid residue by mechanical squeezing.
In the sugar industry, the most widely used pure lixiviation process is that commonly referred to as “diffusion”. In commercial equipment for this process, a bed of the finely divided plant material (typically 1200 to 1800 mm deep) is supported on an elevated perforated screen deck (floor) and moved along from an inlet to a discharge end of the diffuser at a rate of about 1 meter/minute. The bed is contained within a long steel trough of rectangular cross section, usually about 50 to 60 m long (to achieve 50 to 60 minutes retention) and between 5 and 12 m wide (depending on the required throughput capacity), with sides extending upwards to a height of 3 to 5 m. This trough is covered with a light roof.
The bottom of the trough is formed by a (fixed or moving) perforated screen deck, typically having a 20 to 45% open area. The open area may be formed by holes of 10 to 12 mm diameter at a 20 to 25 mm triangular pitch. The bed is transported forward along this deck. As the bed of material progresses along the diffuser, it is subject to lixiviation by flows of liquid percolating through the bed. Typically, the bed passes under sprays of water (at the discharge end of the diffuser) and dilute juice, which percolates through the bed collecting sucrose and down through the perforations in the deck. The liquid flows are applied in a generally counter-current direction to the solid material, with water or low concentration (“low brix”) juice applied at the discharge end of the diffuser and concentrated (“high brix”) juice removed from the inlet end of the diffuser. The juice is then collected in juice trays suspended beneath the screen deck. From these trays, it is pumped forward to further sprays until, at the front of the diffuser, the most concentrated juice is drawn off for further processing.
In existing commercial linear diffusers, the bed of material is transported forward by one of two means. In fixed screen deck diffusers, the bed is dragged forward by slats suspended between a series of chains, driven by sprockets mounted on a single long and costly rotating headshaft. Once the chains pass over the headshaft, they continue under the juice trays back to the feed end of the diffuser.
In moving screen deck diffusers, the bed rides forward on a moving perforated deck. The deck is made in a number of panels the width of the diffuser trough, carried on two large chains at either side of the diffuser. These chains are driven by two large sprockets on a headshaft. The headshaft is rotated either by a rotary drive or by reciprocating hydraulic cylinders that ratchet the chains forward in a discontinuous motion. Once the deck panels pass over the headshaft, they continue under the juice trays back to the feed end of the diffuser.
In both methods, the moving components need to extend across the full width of the diffuser. Both configurations require that, after passing along the diffuser, the bed-transporting components (chain-and-slats or deck panels) need to be returned to the inlet end of the diffuser. Practical disadvantages of these methods are that less than 50% of the costly chain-slat assembly or screen deck is usefully working at any time. Also, to permit the return of the moving components, a clear passage needs to be provided beneath the diffuser. This requires that the deck and body of the diffuser be supported on strong bridge sections across the width of the diffuser. The size and cost of the bridge structures escalates rapidly with the width to be spanned. This limits the maximum throughput capacity of diffusers that can be economically constructed. Moving components of both systems are highly stressed and costly to maintain.
It is an object of the invention to provide an alternative method of transporting a product within a diffuser during a continuous lixiviation process and an alternative transport apparatus.